The present invention relates generally to the measurement of the pulsatile volume and net inflow of fluids in body segments and an analysis of those measurements. More particularly, the present invention relates to the measurement of pulsatile volume and net inflow of blood in the limb of a human, by non-invasive electrical means, and the analysis of that data, together with other physiological data, to enhance the reproducibility and signal-to-noise characteristics of those measurements.
The measurement of peripheral blood flow is important in medicine since there are many specific diseases in which peripheral blood flow is impaired, e.g. diabetes and atherosclerosis. Also, the peripheral blood flow is altered as the total cardiac output is increased or decreased. Thus cardiac output is particularly important in patients who are under anesthesia, are in the post-operative state, or are critically ill or unstable. As blood flow from the heart falls, the peripheral blood flow is dramatically reduced to preserve flow to the brain and vital organs.
Blood flow to an extremity can be measured painstakingly and invasively by dissecting out the main blood vessels to the limb (e.g. brachial artery in an arm) and encircling it with an electromagnetic flow probe. That is clearly not a technique suitable for clinical use. Currently, no means are available for directly measuring peripheral blood flow non-invasively.
The measurement of peripheral blood flow has therefore relied upon techniques which require theoretical assumptions which allow an approximation or calculation of flow from other data which may be obtained non-invasively. The most commonly used techniques involve the calculation of flow from changes in limb volume, a measurement which can be obtained non-invasively. The measurement of volume is referred to as "plethysmography", and includes a variety of techniques including displacement of liquids and gases, and the stretching of mechanical or electrical strain gauges. The present device relates to the family of electrical impedence or admittance plethysmographs in which the body segment itself is an electrical conductor whose properties are altered as it changes in size throughout the cardiac cycle, as blood enters and leaves the body segment.
A change in volume of a body segment may be calculated as follows: ##EQU1## where V=the change in volume of the body segment;
L=the length between the inner measuring electrodes (or length of the body segment); PA0 Z.sub.0 =the baseline impedance; PA0 .rho.=the resistivity of the blood; and PA0 .DELTA.Z=the change in impedance which occurs as the limb or body segment changes size during the cardiac cycle. PA0 By differentiating: ##EQU2## where: dV/dt=the change in volume per unit (or net inflow into the body segment); and PA0 dZ/dt=the rate of change of the impedance.
This formula requires measurement of Z.sub.0, and assumes that Z.sub.0 remains relatively constant throughout the measurement period. That equation may be rearranged as follows: EQU .DELTA.V=.rho.L.sup.2 .DELTA.Y
where Y is the change in admittance. Differentiating: EQU dV/dt=.rho.L.sup.2 (dY/dt)
where (dY/dt) is the rate of change of the admittance. This approach is attractive because the addition or subtraction of blood to the limb or body segment is treated mathematically as the addition or removal of a parallel conductor (blood) to a baseline conductance (the limb or body segment tissue) and there is no need to include that baseline in the calculation.
As stated earlier, no known direct non-invasive technique exists to measure peripheral flow. One technique which has been used over the years and with many different devices is that of venous occlusion plethysmography. By inflating a cuff proximal to the limb segment at a pressure higher than the typical venous pressure, yet lower than the diastolic arterial pressure, it is assumed that the compliant veins will act as a reservoir and expand in volume at the rate of arterial inflow. That technique assumes infinite distensibility of the vein over the first few cardiac cycles post-occlusion and no alteration of arterial inflow due to a low pressure cuff inflation. After a number of cardiac cycles, the venous pressure rises and ultimately exceeds the cuff pressure. The early rate of rise of the volume curve as generated by a plethysmograph is assumed to equal the arterial inflow.
A different approach is to examine the pulsatile volume change and net inflow (or rate of volume change) that occurs within each cardiac cycle, without application of occlusion. The constant component of flow is ignored and instead, the focus is on the pulsatile component. That approach relies upon the assumption that total flow varies with pulsatile flow. Although that introduces theoretical inaccuracies into the absolute measurement of flow, it does preserve the ability to follow changes and trends in flow. The admittance technique has not yet been applied to that type of measurement.
When measuring pulsatile flow, since the body segment volume changes less than 0.05% with each cardiac cycle, such technique is severely limited by the signal-to-noise characteristics of the electronic device used. Furthermore, there is considerable beat-to-beat variability of the peripheral volume pulse making that measurement difficult to reproduce.
The volume of an electrical conductor may be determined from its electrical conductance. At high frequencies (e.g. between 20 and 100 kilohertz), biological tissues behave essentially as pure conductors. That principle may be used to measure volume changes occurring in a body segment. Such a technique, usually referred to as "electrical admittance (or impedance) plethysmography," has been used to estimate peripheral limb or body segment flow utilizing the venous occlusion method described above. That method involves the use of a cuff, which is attached around the limb whose flow is to be monitored, and then inflated in order to occlude the venous blood flow in that limb.
One such device which utilizes the venous occlusion method for measuring blood flow rate non-invasively is disclosed in U.S. Pat. No. 4,204,545, to Yamakoshi. In that device, the initial admittance value of the limb is measured and then the venous return in the limb to be examined is occluded. The initial admittance value of the limb is held, and then compared with the subsequent admittance value of the limb due to the venous occlusion. The device of Yamakoshi utilizes an automatic cuff to accomplish the venous occlusion and four electrodes spaced apart on the limb to be examined. A high-frequency, very small current is supplied to the limb segment and the limb blood flow is measured as a change in the electrical admittance of the limb segment caused by the venous occlusion. While that method enables a non-invasive measurement of blood flow, the device disclosed therein erroneously assumes that the output amplitude of the input current device remains constant.
While Yamakoshi has applied the admittance technique and an automatic cuff inflation device to the computation of venous occlusion plethysmograms, he has computed admittance simply by taking the reciprocal of impedance and assumed that the exciting voltage was accurate and non-varying. Because the venous occlusion method utilizes relatively large pooling phenomenon, and relies upon a straight line approximation to a volume curve which has superimposed pulsatile components, the precise measurement of small volume changes is not imperative when using that technique.
An impedance plethysmograph in which an analog computer is used in order to compute the blood flow from the measured blood volume changes is disclosed in U.S. Pat. No. 3,835,839, to Brown. In that patent, a flow rate computer is disclosed which operates with an impedance plethysmograph of the type having current and voltage electrodes which are applied to a selected biological segment. Plethysmograph outputs are generated which are proportional to tissue conductance within the segment being examined, the deviation from the basic resistance of the segment and the rate of change of that deviation. The computer analyzes such signals and, together with constants of proportionality injected into the circuit at appropriate points, provides an output in units of absolute flow rate per unit of time.
Since limb admittance varies less than 0.05% during the cardiac cycle, that rather small change causes the extraction of the signal from the background noise to be a technically difficult problem. Thus, the use of the limb admittance (or impedance) plethysmogram has been almost exclusively limited to the venous occlusion technique described by Yamakoshi, and others, in which relatively large volume changes occur over multiple beats.
However, in order to provide a more accurate peripheral volume pulse measurement than the prior art, such as the Brown device, the instant admittance plethysmograph was developed. By improving the signal-to-noise ratio utilizing field effect transistor circuitry to stabilize the signal source and an analog divider in order to correct for fluctuations of the input amplitude, the instant device provides an improved and accurate means for measuring the peripheral limb pulsatile volume and net inflow. Details of the individual volume pulse wave measured by the instant admittance plethysmograph are clarified by signal averaging, utilizing a digital computer system.